And we begin with breaking news. The report of a shooting at ft. Hood in Texas. The nation's largest army base. Ft. Hood, the site of that other deadly shooting nearly five years ago. And now, tonight, word of a gunman on the loose. And here's the message tweeted out to the people on base. They are told to shelter in place. Details are pouring in. And ABC's chief justice correspondent, Pierre Thomas, has the latest right now. Pierre, what are you hearing? Reporter: Diane, the FBI tells us there has been a shooting at the ft. Hood military post in Texas. There's reports of casualties. But we don't know how many people have been injured. There's reason to be believed that there's an active shooter. Military officials have issued a tweet telling all personnel to shelter in place. There's great concern because of the recent tragic history at that military installation. It comes roughly five years after army major Nadel Hasan opened fire at ft. Hood, killing 13 people and wounding 32 others. It was the worst mass murder at a military installation. Major Hasan has been sentenced to death. And again, to get what you know at this point. The gunman, apparently, is still on the move, as far as we know. Reporter: The gunman, not in custody. This is active event, still ongoing, we're being told. Reports of casualties. We're trying to get firm Numbers. A very active, dangerous situation. And that is a huge number of people, now told to take hiding, to shelter in place. And I want to go, now, to chief investigative reporter, Brian Ross, also making calls tonight. What are you hearing? Reporter: Here's what we do know. It's surprisingly easy to get on a U.S. Military base, a U.S. Army base. Sometimes cars aren't even checked. In the last two years, U.S. Law enforcement and counterintelligence authorities have said that homegrown jihadists have tried to focus on military targets, not civilian targets. We don't know who is involved in this shooting tonight. But we do know in the last two years, the FBI has had under scrutiny over 100 people in the service, who are considered possible insider threats. This is a very real concern and has been for years. We would have thought after five years ago, it would have been the tightest possible security. Well, in a congressional hearing two years ago, there was testimony that the army was slow to adopt procedures to help identify possible insider threats. There remains great concern about that inside the military. Let's check with Martha Raddatz in Washington right now. You've been to ft. Hood so often, Martha. Reporter: Diane, in fact, I'm supposed to be in ft. Hood this weekend. So, I've been in touch with a lot of people over the last week about what was going on this weekend. In fact, they made sure that they had our I.D. Information to get on the base. So, I think it's sometimes very strict and other times it's not. At this point, we have no idea what's going on with the active shooter. We do know confirmed that there is an active shooter, a reminder that this is the largest army base in the country. The 1st cavalry division is there. The 4th infantry division. Both have served numerous times in Iraq, in Afghanistan, again and again. And as you know, so many deployments for those soldiers over and over and over again. Just this afternoon, Diane, I was talking to a medical officer out of the Pentagon about the stressors on the force. About mental health issues with the 4th and what they're doing to try to help soldiers. We have no idea, again, what is going on or who is involved. Martha, Brian, Pierre, all at their posts, monitoring every message. I want to put up once again what was tweeted to everyone on the base. All personnel on post are asked to shelter in place. And we will tell you everything we are learning, as we learn it, throughout this broadcast.

This transcript has been automatically generated and may not be 100% accurate.

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